Use of Superconductors as Thermometric Fixed Points

Abstract
The possibility of using superconductors with sharp transition temperatures as fixed points on a thermometer scale was examined. Annealed polycrystalline specimens 3.2 cm long by 0.157 cm in diameter were placed in a cryostat which would operate from 0.3 to 10.0 K; the superconductive transitions were measured with a Hartshorn bridge operating at 270 Hz. One specimen each was made from the purest available Pb, In, Ga, Zn, and Cd—the latter two elements are obtainable from the Office of Standard Reference Materials. In all cases but Cd, the width of the transitions were less than 0.001 K; for Cd it was 0.002 K. The specimens were cycled once to room temperature and remeasured to check their reproducibility vs two germanium resistance thermometers. The reproducibility was better than 0.001 K. These preliminary results indicate the strong possibilities for this approach to fixed point thermometry.